When filling the tank of a motor vehicle, it is a common measure to recover the vapor escaping the tank when filling it with liquid fuel from a fuel pump unit. The liquid fuel is usually drawn from an underground fuel reservoir. The vapor recovery is performed by a system which is incorporated in the fuel pump unit, such that vapor of hydrocarbons are prevented from escaping to the environment outside the tank of the motor vehicle. Such a vapor recovery system typically consists of a vapor pump, a flow meter and a vapor inlet arranged at a fuel dispensing nozzle. The recovered vapor is returned to the underground fuel reservoir. The vapor pump draws fuel vapor from the tank of the motor vehicle, through the vapor flow meter and to the fuel reservoir. The flow meter constantly measures the vapor flow and compares the measured value with the flow of dispensed fuel. Adjusting means are connected to the flow meter and to the vapor pump, and are configured to control the rate of vapor flow so that it matches the flow rate of the dispensed fuel.
It is important that the vapor flow rate corresponds to the fuel rate, and, accordingly, there is a need for a reliable way to check that vapor flows into the vapor tank while dispensing fuel.
There are several techniques for monitoring a vapor flow.
EP-1,101,728, for example, discloses a fuel dispensing system which includes vapor flow and hydrocarbon concentration sensors positioned in a vapor line, normally the vapor recovery line, to provide accurate feedback relating to the vapor flow rate and concentration of hydrocarbon laden vapor recovered by a vapor recovery system. The sensors provide diagnostic information about the vapor recovery process as well as ensure that the vapor recovery process is carried out in an efficient manner.
U.S. Pat. No. 6,418,981 discloses a dispensing installation for fuel or other volatile liquids which comprises a storage tank, a pipe and a pump for dispensing fuel, a vapor recovery pipe and pump for recovering vapor and delivering it back to the tank, and a controller maintaining the vapor delivery rate approximately equal to the liquid delivery rate. In order to check that the vapor recovery system is operating correctly, the vapor delivery rate is constantly detected and compared with a value of the liquid delivery rate. If the result of the comparison is outside a predetermined range, an alarm is triggered in order to indicate a malfunction.